1. Field of the Invention
This invention relates to regulator circuits and, more particularly, to a PNP series shunt integrated regulator circuit having a low input/output voltage differential.
2. Description of the Prior Art
In an attempt to improve the gas mileage of today's internal combustion engines, the automobile industry is increasingly relying on monolithic integrated circuit electronic ignition systems for controlling engine sparking. Generally, the automobile manufacturer has required these ignition systems to be able to operate with a battery operating potential that can vary between five to 80 volts, conditions which correspond to low battery starting and load dump respectively. This requirement has necessitated the use of a regulator circuit internal to the solid state ignition circuit to provide a constant supply voltage to operate the individual components and circuitry of the ignition system.
One type of regulator circuit used in some prior art ignition systems is the shunt regulator which includes a resistor coupled in series with a zener diode both of which are fabricated internally to the integrated ignition circuit with the resistor being coupled via an external terminal to the car battery. The regulated voltage is taken from across the Zener diode to operate the circuitry comprising the ignition system. A problem arises with this type of regulator in that a minimum supply voltage and load current is required to operate the ignition system which, typically, are four volts and between three to ten milliamps. In order to provide four volts when the car battery is five volts the series resistor can be no larger than 100 ohms if ten millamps of current is to be provided. Thus, assuming that a ten volt Zener diode is used for regulation, under the 80 volts load dump condition a minimum of 700 milliamps will flow therethrough which means that 7 watts must be dissipated under the load dump condition by the Zener. This is very undesirable as the integrated circuit cannot be safely operated. Therefore, there is a need to replace the shunt regulator as used in some prior art ignition system with some other type of regulator circuit.
A series pass NPN transistor type of regulator, although not suffering from the power dissipation problems of the shunt regulator, requires a NPN device having both a large BV.sub.CEO breakdown and a low input/output voltage characteristic which are not easily obtained using today's integrated circuit processing techniques. Therefore, in order to withstand the 80 volts supply voltage under load dump conditions which requires a high V.sub.CEO breakdown, the input to output differential of the series pass device due to the V.sub.CE sat characteristics and base current drive component thereof at 5 volts is not low enough to provide the 4 volts minimum to the integrated ignition system.
Thus, a need exists for a low input/output differential regulator circuit that is suitable to be fabricated in monolithic integrated circuit form and which can be operated from an unregulated potential that varies over a wide range in magnitude while both supplying a minimum output voltage at an output thereof and dissipating minimal power.